To keep cross-compiling straightforward, Cretonne shouldn't have any
behavior that depends on the host. This renames the "Native" calling
convention to "SystemV", which has a defined meaning for each target,
so that it's clear that the calling convention doesn't change
depending on what host Cretonne is running on.
The spiller wasn't tracking register pressure correctly for dead EBB
parameters in visit_ebb_header(). Make sure we free any dead EBB
parameters.
Fixes#223